This invention relates to an arc fault detection device, and in particular, to an arc fault detection device that uses two components of an arcing waveform.
A percentage of fires each year are caused by electrical branch circuit wiring arcing faults involving currents below the trip level of a conventional circuit breaker or OCPD (over current protection device) as well as below the handling rate of the breaker. Basic over current protection afforded by circuit breakers is designed to prevent I2R heating of the wiring in the electrical distribution system, caused by circuit overloading or line-to-line faults, and not necessarily arcing faults. A true short circuit is a rarity in an electrical system. In fact, it is more accurate to think of electrical faults as having some level of impedance, such as a high impedance arc fault (low current) or a low impedance fault (high current). Many electrical faults begin as high impedance breakdowns between the line and neutral conductors or to the ground wire or device components. AFCI (Arc Fault Circuit Interruption) technology affords protection from conditions that may not necessarily be an immediate threat but could become hazardous if left unattended.
In order to start a fire, three elements must be present fuel, oxygen(air), and energy to ignite the fuel. Arcing is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can reach temperatures of several thousand degrees Celsius. Arcing produces sufficient energy to reach the ignition point of nearby combustible material(s) before a circuit breaker can respond. Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers or OCPD""s, which alone may not detect and respond to arc faults.
A number of devices for detecting arc faults and methods of detection have been used in the past. These include using E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, using non-overlapping band pass filters to detect white noise characteristic of arcs, and utilizing the high frequency components (RF) of arcing waveforms to detect arcing faults. While some of these techniques are more or less effective than others, they require relatively sophisticated arc sensors and circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers.
There is a need for simple economical arc fault detectors that can be included in wiring devices such as duplex receptacles, multi-outlet strips, or in-line devices, and that offer the same downstream protection as an arc fault detector incorporated in a circuit breaker but at lower cost. There is a need for an arc fault circuit detector in wiring devices that can be provided at a reduced cost compared with arc fault circuit detecting circuit breakers comparable to the reduction in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers.
Arc fault detection technology responds to arcing faults by looking at specific arc xe2x80x9csignaturexe2x80x9d characteristics unique to arcing faults. This invention provides an arc fault circuit interrupter, preferably contained in a receptacle embodiment, that detects parallel or line to line (LINE to NEUTRAL or LINE to GROUND) arcs, producing currents of 75 amps or greater. The detection of parallel arcs is a requirement in Underwriters Laboratories Standard 1699 for duplex receptacle AFCI""s known as combination devices. The requirements of UL-1699 state that an arc fault protection device must detect an arc within eight arcing half cycles at 75 Amperes or greater.
Briefly stated, an arc fault detector for detecting arcing faults in an electrical distribution/branch circuit includes both a di/dt detector/amplifier and a 60 Hz (threshold) detector/amplifier connected to a current sensor. The detector includes a circuit which masks inductive/tungsten in-rush current such as is generated by a dimmer switch. The current sensor is a toroidal current transformer, with either two LINE conductors or a LINE and a NEUTRAL conductor passing through the toroid. A multi-winding secondary is formed around the toroidal core whose terminations constitute the output of the current sensor. When the outputs of both detectors simultaneously produce signal, a circumstance that is produced by a phase control dimmer, a breaker coil is energized which in turn de-energizes the electrical distribution/branch circuit to terminate the parallel arc fault.
According to an embodiment of the invention, an arc fault detector for an AC power line, where the AC power line includes first and second conductors for carrying current to a load, includes a current sensor coupled to the first and second conductors for generating a signal responsive to rate of change and threshold behavior of the current in the conductors; first and second detectors receiving the signal from the current sensor as inputs; the first detector detecting the rate of change behavior of the current and outputting a first pulse; the second detector detecting the threshold characteristics of the current and outputting a second pulse; first and second pulse wideners for widening the first and second pulses, respectively; a dimmer delay circuit receiving the first pulse as input and outputting a pulse having a predetermined delay and pulse width; a first logic circuit receiving the widened first and second pulses and producing a pulse output indicative of both widened first and second pulses being present at the first logic circuit simultaneously, and a second logic circuit receiving the output from the first logic circuit and the pulse from the dimmer delay circuit and producing a pulse output indicative of both the pulse output from the first logic circuit and the pulse from the dimmer delay circuit being present at the second logic circuit simultaneously.
According to an embodiment of the invention, an arc fault detector for an AC power line where the AC power line includes first and second conductors for carrying current to a load includes a current sensor coupled to the first conductors for generating a signal responsive to rate of change behavior of the current in the conductor; a resistive element in series with the second conductor; a first detector receiving the signal from the current sensor as input; a second detectors receiving a signal from a load side of the resistive element as input; the first detector detecting the rate of change behavior of the current and outputting a first pulse; the second detector detecting a threshold characteristic of the current and outputting a second pulse; first and second pulse wideners for widening the first and second pulses, respectively; a dimmer delay circuit receiving the first pulse as input and outputting a pulse having a predetermined delay and pulse width; a first logic circuit receiving the widened first and second pulses and producing a pulse output indicative of both widened first and second pulses being present at the first logic circuit simultaneously, and a second logic circuit receiving the output from the first logic circuit and the pulse from the dimmer delay circuit and producing a pulse output indicative of both the pulse output from the first logic circuit and the pulse from the dimmer delay circuit being present at the second logic circuit simultaneously.
According to an embodiment of the invention, an arc fault detector for an AC power line, where the AC power line includes first and second conductors for carrying current to a load, includes means, coupled to the first and second conductors, for generating a signal responsive to rate of change and threshold behavior of the current in the conductors; means for detecting the rate of change behavior of the current and outputting a first pulse; means for detecting the threshold characteristics of the current and outputting a second pulse; means for widening the first and second pulses, respectively; delay means for receiving the first pulse as input and outputting a pulse having a predetermined delay and pulse width; first logic means for receiving the widened first and second pulses and producing a pulse output indicative of both widened first and second pulses being present at the first logic means simultaneously; and second logic means for receiving the output from the first logic means and the pulse from the delay means and producing a pulse output indicative of both the pulse output from the first logic means and the pulse from the delay means being present at the second logic means simultaneously.
According to an embodiment of the invention, a method for detecting an arc fault in an AC power line, where the AC power line includes first and second conductors for carrying current to a load, includes the steps of: (a) generating a signal responsive to rate of change and threshold behavior of the current in the conductors; (b) detecting the rate of change behavior of the current and outputting a first pulse; (c) detecting the threshold characteristics of the current and outputting a second pulse; (d) creating a third pulse similar to the first pulse but having a predetermined delay and pulse width; (e) producing a fourth pulse indicative of both the first and second pulses being present at a first logic point simultaneously; and (f) producing a fifth pulse indicative of both the third and fourth pulses being present at a second logic point simultaneously.
According to an embodiment of the invention, a method for detecting an arc fault in an AC electrical distribution system includes the steps of detecting a sharply rising edge in current flowing through a portion of the AC electrical distribution system, detecting an increase in magnitude of the current above a predetermined threshold level, and establishing that the sharply rising edge in the current and the increase in magnitude of the current are not caused by an inrush current from a device.
According to an embodiment of the invention, a method for detecting an arc fault in an electrical distribution system includes the steps of monitoring a load current and a time derivative of the current carried by the electrical distribution system; creating first and second signals from the step of monitoring; independently comparing magnitudes of the first and second signals against pre-established thresholds; creating first and second asynchronous pulses from the step of comparing magnitudes; elongating the first and second asynchronous pulses by first and second pre-determined durations; opening electrical contacts in the electrical distribution system if the elongated pulses overlap; and interrupting a flow of electrical current through the electrical distribution system upon an overlapping occurrence.
According to an embodiment of the invention, a method for detecting an arc fault in an electrical distribution system, wherein the electrical distribution system includes a plurality of conductors, includes the steps of monitoring a load current and a time derivative of current carried by the conductors; creating first and second signals from the step of monitoring; independently comparing magnitudes of the first and second signals against preestablished thresholds; creating first and second asynchronous pulses from the step of comparing magnitudes; initiating a pre-determined time interval when the first signal first exceeds the threshold; elongating the first and second asynchronous pulses by first and second pre-determined durations; and opening electrical contacts in the electrical distribution system if the elongated pulses overlap and only after the pre-determined time interval has elapsed.
According to an embodiment of the invention, a method for detecting an arc fault in an electrical distribution system, wherein the electrical distribution system includes a plurality of conductors, includes the steps of discriminating between arcing faults and phase control dimmer signals; producing a pulse elongated by a pre-determined time whenever a magnitude of a time first derivative of a current carried on the electrical conductors exceeds a threshold; initiating a predetermined time interval when the pulse is detected; generating a trigger signal that triggers a set of load current contacts to open when the pulse is detected; disabling the trigger signal when the current is below a threshold; and disabling the trigger signal if the pulse occurs during the pre-determined time interval.